Pattern mask with features to minimize the effect of aberrations

ABSTRACT

A semiconductor pattern mask that might otherwise exhibit three-fold symmetry, which could give rise to distorted semiconductor features in the presence of three-leaf aberration in the optical system used to expose a semiconductor wafer through the mask, is altered to break up the three-fold symmetry without altering the semiconductor features that are formed. This accomplished by adding features to the mask that break up the symmetry. One way of achieving that result is to make the added features of “sub-resolution” size that do not produce features on the exposed wafer. Another way of achieving that result is to change existing features that do form structures in such a way (e.g., with optical elements) that changes the relative phase, amplitude or other characteristic of light transmitted through those features.

BACKGROUND OF THE INVENTION

[0001] This invention relates to a semiconductor pattern mask havingfeatures to minimize aberrations. More particularly, this inventionrelates to a semiconductor pattern mask having sub-resolution featuresto reduce the sensitivity of the pattern to aberrations in the optics ofthe pattern imaging system.

[0002] Semiconductor devices are typically manufactured usingphotolithographic techniques. The circuit elements or structures to beformed are drawn on a mask. The mask can be a “dark field layout” inwhich the circuit elements are represented by light-transmissive areason a nontransmissive (or less transmissive) background, or a “clearfield layout” in which the circuit elements are represented bynontransmissive (or less transmissive) areas on a transmissivebackground.

[0003] A silicon substrate, suitably doped, is provided with aphotosensitive coating or “photoresist.” The photosensitive coating isexposed to light through the mask using an optical system, and is thenprocessed to develop the circuit elements on the silicon substrate. Theprocess is repeated for multiple layers of silicon and metallization(using a different mask for each layer) until the desired circuit hasbeen formed.

[0004] The optical elements in the optical system used to expose thephotosensitive surface through the mask may be imperfect. For example,lenses in that system may be manufactured with one of several opticalaberrations.

[0005] One such aberration, known as three-leaf aberration, may causedistortion of the imaging of the mask features onto the photosensitivesurface, resulting in corresponding distortions in the finalsemiconductor device.

[0006] It would be desirable to be able to provide a way to eliminatethe sensitivity of semiconductor pattern mask to three-leaf aberration.

SUMMARY OF THE INVENTION

[0007] Preferably, in accordance with the present invention, asemiconductor pattern mask includes first features intended to formstructures in the semiconductor end product, and second features thatdiffer from such first features in a way that breaks up three-foldsymmetry in the pattern, and may or may not form structures in thesemiconductor end product. The latter features may be of differenttransmissivity than the former features, which results in breaking upthe three-fold symmetry. In one embodiment, the differing transmissivityis a result of a difference in the size of the features, with additionalfeatures being smaller than the features intended to form structures.The smaller features are not intended to form structures, but still arelarge enough to affect the pattern of transmitted light. In anotherembodiment, certain features may transmit light at a relative phasedifferent than that of the light transmitted by other features. Thecertain features may be of substantially the same size as the otherfeatures, both of which being intended to form structures in thesemiconductor end product, or the certain features may be smaller thanthe other features, and not intended to form structures in thesemiconductor end product.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] The above and other objects and advantages of the invention willbe apparent upon consideration of the following detailed description,taken in conjunction with the accompanying drawings, in which likereference characters refer to like parts throughout, and in which:

[0009]FIG. 1 is a plan view of a portion of semiconductor pattern maskthat is susceptible to three-leaf aberration;

[0010]FIG. 2 is a plan view of the light pattern expected to betransmitted by the mask of FIG. 1 to a photosensitive semiconductorwafer surface;

[0011]FIG. 3 is a plan view of the light pattern actually transmitted bythe mask of FIG. 1 in the presence of three-leaf aberration;

[0012]FIG. 4 is a plan view of the diffraction pattern transmitted bythe mask of FIG. 1;

[0013]FIG. 5 is a plan view of one preferred embodiment of asemiconductor pattern mask according to the present invention;

[0014]FIG. 6 is a plan view of the diffraction pattern transmitted bythe mask of FIG. 5;

[0015]FIG. 7 is a plan view of the light pattern actually transmitted bythe mask of FIG. 5 in the presence of three-leaf aberration;

[0016]FIG. 8 is plan view of another preferred embodiment of asemiconductor pattern mask according to the present invention; and

[0017]FIG. 9 is a plan view of a third preferred embodiment of asemiconductor pattern mask according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0018] The features on a semiconductor pattern mask are small enoughthat when light passes through the mask, diffraction occurs at thelocations of the features. This is generally the case whether the maskis a dark-field mask, in which case the diffraction occurs as lightpasses through the relatively small openings that represent features inthe final semiconductor, or a clear-field mask, in which casediffraction occurs around the edges of the relatively smallnontransmissive areas that represent features in the finalsemiconductor.

[0019] The light that passes through a semiconductor pattern mask isdirected via suitable optics, including lenses, onto the photosensitivesurface of a semiconductor substrate. Certain optical defects in thoselenses may give rise to aberrations that distort the image formed by thelight on the substrate surface, and therefore distort the featuresformed on the semiconductor. The defects may affect only a portion of alens, so that not all light passing through that lens is distorted, butonly light that happens to pass through the affected portion. Moreover,not every lens in the system, even among lenses manufactured atsubstantially the same time and place, will have the defect in the sameplace, if at all. Therefore, the pattern of the mask cannot be“pre-distorted” so that the “distorted” image is actually the desiredimage, because the distortion in the optical system may be different foreach piece of equipment with which identical masks are used.

[0020] As discussed above, one such aberration is known as “three-leafaberration.” Three-leaf aberration is known to cause distortion inimages formed from masks having three-fold symmetry.

[0021] Thus, masks that have a substantially uniform distribution offeatures, even if not all of those features are identical in size ortransmissivity, are substantially unaffected by three-leaf aberration.Similarly, masks that are nonuniform, but whose features aresubstantially spaced apart from one another, are substantiallyunaffected by three-leaf aberration.

[0022] The types of masks, then, that are affected are those with dense,nonuniform patterns. For example, a dense pattern having featuresarranged in groups of three would have three-fold symmetry and besubject to three-leaf aberration.

[0023] In accordance with the present invention, the effects ofthree-leaf aberration can be eliminated for such a mask by making itappear that the mask is one having a dense uniform pattern. That can beachieved by providing on the mask, in locations where there wouldotherwise be no features, features that are too small to form featureson the semiconductor substrate. Nevertheless, these “sub-resolution”features, properly sized as discussed below, are large enough to affectthe diffraction pattern and to break the three-fold symmetry of themask.

[0024] Thus, in a dark-field mask having groups of, e.g., three openingson an opaque background, separated by areas of opaque background ofsizes on the same order as the openings, small openings could beprovided in those background areas to break up the three-fold symmetryand avoid three-leaf aberration. Similarly, in a clear-field mask havinggroups of, e.g., three dark areas on a transmissive background,separated by areas of transmissive background of sizes on the same orderas the dark areas, small dark areas could be provided in thosebackground areas to break up the three-fold symmetry and avoidthree-leaf aberration. These small areas, properly sized as discussedbelow, are large enough to break the three-fold symmetry, but smallenough that they do not produce features on the semiconductor substrate.

[0025] Instead of providing small openings or small dark areas, it ispossible to alter some of the mask features intended to form structureson the semiconductor wafer, so that they differ in transmissivity fromunaltered mask features in a way that breaks up the three-fold symmetryof the mask. For example, the mask features to be altered could beformed in such a way that they transmit light with a different relativephase as compared to the unaltered mask features. Both altered andunaltered mask features in this embodiment would form structures on thesemiconductor wafers, but the three-fold symmetry susceptible tothree-leaf aberration would be broken. One way of doing this in adark-field mask, for example, where the mask features may be simpleopenings, is to provide, in each opening whose transmissivity is to bealtered, a lens, filter or other optical element that changes therelative phase of light passing through it. Alternatively, the quartzplate normally present behind the mask can be etched, in the areasbehind the openings whose transmissivity is to be changed, in such a wayas to change the relative phase of light passing through those openings.

[0026] In another embodiment, instead of altering transmissivity byaltering phase, the amplitude or intensity of the transmitted lightcould be altered. Thus, in a dark-field mask, the openings whosetransmissivity is to be altered could be provided with optical elementsthat reduce the amplitude or intensity of light transmitted, while in aclear-field mask, the dark areas whose transmissivity is to be alteredcould be lightened to increase the amplitude or intensity of transmittedlight. Again, at least in the dark-field case, the quartz plate behindthe mask could be darkened in the areas where transmissivity is to bealtered so as to reduce the intensity or amplitude of transmitted light.

[0027] It should also be noted that the provision of some structure toaffect the phase of transmitted light can be combined with the use ofsmaller—i.e., sub-resolution—features as described above. The onlyrequirement is that the resulting transmitted light not result in theformation of any structure on the semiconductor wafer, while still beingsufficient to break up the three-fold symmetry of the mask pattern,which in turn avoids the effects of three-leaf aberration.

[0028] The invention will now be described with reference to FIGS. 1-9.

[0029]FIG. 1 shows a portion of a semiconductor pattern mask 10 that issusceptible to three-leaf aberration. Mask 10 as depicted represents acontact layer of a multi-layer semiconductor, and embodies a dark-fieldlayout, described above, preferably having a background 11 of a first,lower transmissivity, and feature openings 12 in background 11, eachpreferably having a second transmissivity higher than the firsttransmissivity. Note that the pattern shown on mask 10 has three-foldsymmetry, being made up of groups of three openings 12 separated byareas 13 of background 11.

[0030]FIG. 2 shows the pattern 20 of light intended to be transmitted bymask 10 onto a semiconductor wafer. The pattern shown is in fact thepattern transmitted in the absence of three-leaf aberration. As shown,each feature opening 12 forms a substantially circular image 21. Theeffects of three-leaf aberration on pattern 20 can be seen in FIG. 3,where one or more images 31 of pattern 30 is distorted as compared tocorresponding images 21 of pattern 20. In particular, each image 310 asdepicted is elongated in the vertical direction at 311 as compared witha corresponding image 210 at 211 in FIG. 2.

[0031] As explained above, the distortion effects visible in FIG. 3result from the presence of three-fold symmetry in pattern 20. FIG. 4shows the diffraction pattern 40 produced by pattern 20, in whichfeatures 41 are the result of three-fold symmetry.

[0032] In accordance with one preferred embodiment of the presentinvention, a modified pattern mask 50 may be provided, as shown in FIG.5, to form the pattern that mask 10 is intended to form. Mask 50preferably includes openings 51 similar to openings 11, similarly spacedand grouped, but in areas 52, where mask 10 has empty dark areas, mask50 preferably has sub-resolution openings 53—i.e., as explained above,openings that are sufficiently smaller than openings 51 that they do notresult in the formation of structures on a semiconductor wafer that isexposed through mask 50.

[0033] For example, in one embodiment, the dimensions of the centeropening 51 in each group of three openings 51 are preferably about 200nm (horizontal dimension) by about 210 nm (vertical dimension), and thedimensions of the upper and lower openings 51 in each group of threeopenings 51 are preferably about 190 nm (horizontal dimension) by about215 nm (vertical dimension). In an embodiment in which such a mask isexposed using deep-ultraviolet light at a wavelength of 248 nm (such asmight be provided by a krypton fluoride excimer laser), if the numericalaperture of the system were about 0.70, using annular illumination withan outer value of about 0.8 and inner value of about 0.5, the minimumdimensions of the smaller, sub-resolution areas preferably would bebetween about 70 nm and about 80 nm, below which they might have someeffect, but would not have a sufficiently significant effect in breakingup the three-fold symmetry and correcting for the aberration, and themaximum dimensions would be about 150 nm, above which they may no longerbe sub-resolution, and might start producing features on the substrate.The preferred dimensions in this particular embodiment are between about100 nm and about 150 nm.

[0034] Generally, for systems in which the numerical aperture is betweenabout 0.70 and about 0.80, the dimensions of the sub-resolution featurespreferably would be between about one-third and about one-half of thewavelength used. As the numerical aperture increases, the dimensionsdecrease. Therefore, if one were to develop a system having a lens witha numerical aperture greater than 0.8, the dimensions of thesub-resolution features might be less than one-third of the wavelength.Many factors affect the dimensions of the sub-resolution features for aparticular system, including wavelength, numerical aperture, size of themask features intended to form features on the substrate, transmissivityof the mask, etc. Therefore, while as a rule of thumb one might expectthe dimensions of the sub-resolution features to be between aboutone-third and about one-half of the wavelength, every system will bedifferent. However, one of ordinary skill will be able to determine theappropriate dimensions with minimal experimentation.

[0035] Although openings 53 are too small to form features, they arelarge enough that their presence breaks up the three-fold symmetry ofmask 50. In the example above, this was true as long as the dimensionsof the openings were at least between about 70 nm and about 80 nm. FIG.6 shows the diffraction pattern 60 produced when light is passed throughmask 50. As can be seen, diffraction pattern 60 is substantiallyidentical to diffraction pattern 40 without the features 41 resultingfrom three-fold symmetry. And as seen in FIG. 7, the resulting lightpattern 70 that would be imaged onto a semiconductor wafer, even in thepresence of three-leaf aberration, is substantially identical to pattern20 of FIG. 2, without the distortions shown in FIG. 3.

[0036] In another preferred embodiment 80, as shown in FIG. 8, insteadof providing additional openings, such as openings 53, particular ones810 of openings 81 (similar to openings 11, 51), may be provided in sucha way that the phase of light transmitted through them is altered. Forexample, a lens, filter or other optical element 84 that changes therelative phase, or the intensity or amplitude, of light passing throughit could be provided at each feature 810. The degree to which thetransmissivity of features 810 can or should be changed depends in parton the photosensitivity of the photoresist material. While thesefeatures 810 participate in the formation of structures on thesemiconductor wafer, they also break up the three-fold symmetry.

[0037] Moreover, in yet another preferred embodiment shown in FIG. 9, apattern 90 can be used in which features 93 for breaking up thethree-fold symmetry have smaller dimensions than openings 91 and alsoprovided in such a way that the phase of light transmitted through themis altered.

[0038] Thus it is seen that a semiconductor pattern mask whosesensitivity to three-leaf aberration is reduced or eliminated isprovided. One skilled in the art will appreciate that the presentinvention can be practiced by other than the described embodiments,which are presented for purposes of illustration and not of limitation,and the present invention is limited only by the claims which follow.

What is claimed is:
 1. A semiconductor pattern mask comprising: a masksubstrate characterized by a first transmissivity to light; a pluralityof first features on said mask substrate for forming structures in asemiconductor pattern, each of said first features being characterizedby a second transmissivity to light; and a plurality of second featureson said mask substrate, each of said second features being characterizedby a third transmissivity to light.
 2. The semiconductor pattern mask ofclaim 1 wherein said first transmissivity is substantially opaque andsaid second transmissivity is substantially transmissive.
 3. Thesemiconductor pattern mask of claim 1 wherein said first transmissivityis substantially transmissive and said second transmissivity issubstantially opaque.
 4. The semiconductor pattern mask of claim 1wherein: said third transmissivity is substantially equal to said secondtransmissivity; and each of said second features is smaller than any ofsaid first features, said second features not forming structures in saidsemiconductor pattern.
 5. The semiconductor pattern mask of claim 4wherein: each of said first features has dimensions selected to beimaged by light having a predetermined wavelength; and each of saidsecond features has dimensions substantially less than saidpredetermined wavelength.
 6. The semiconductor pattern mask of claim 5wherein each of said second features has dimensions of between aboutone-third and about one-half of said predetermined wavelength.
 7. Thesemiconductor pattern mask of claim 1 wherein said second and thirdtransmissivities differ in relative phase.
 8. The semiconductor patternmask of claim 7 wherein each of said second features is at least similarin size to said first features, said second features forming structuresin said semiconductor pattern.
 9. The semiconductor pattern mask ofclaim 7 wherein each of said second features is smaller than any of saidfirst features, said second features not forming structures in saidsemiconductor pattern.
 10. The semiconductor pattern mask of claim 1wherein at least a subset of said first features are in a nonuniformrepetitive pattern.
 11. The semiconductor pattern mask of claim 10wherein said nonuniform repetitive pattern is dense.
 12. Thesemiconductor pattern mask of claim 10 wherein: said thirdtransmissivity is substantially equal to said second transmissivity;each of said first features has dimensions selected to be imaged bylight having a predetermined wavelength; and each of said secondfeatures has dimensions substantially less than said predeterminedwavelength, said second features not forming structures in saidsemiconductor pattern.
 13. The semiconductor pattern mask of claim 12wherein each of said second features has dimensions of between aboutone-third and about one-half of said predetermined wavelength.
 14. Thesemiconductor pattern mask of claim 10 wherein said second and thirdtransmissivities differ in relative phase.
 15. The semiconductor patternmask of claim 14 wherein each of said second features is at leastsimilar in size to said first features, said second features formingstructures in said semiconductor pattern.
 16. The semiconductor patternmask of claim 14 wherein each of said second features is smaller thanany of said first features, said second features not forming structuresin said semiconductor pattern.
 17. The semiconductor pattern mask ofclaim 10 wherein substantially all of said first features are in saidnonuniform repetitive pattern.
 18. The semiconductor pattern mask ofclaim 17 wherein said nonuniform repetitive pattern is dense.
 19. Thesemiconductor pattern mask of claim 17 wherein: said thirdtransmissivity is substantially equal to said second transmissivity;each of said first features has dimensions selected to be imaged bylight having a predetermined wavelength; and each of said secondfeatures has dimensions substantially less than said predeterminedwavelength, said second features not forming structures in saidsemiconductor pattern.
 20. The semiconductor pattern mask of claim 19wherein each of said second features has dimensions of between aboutone-third and about one-half of said predetermined wavelength.
 21. Thesemiconductor pattern mask of claim 17 wherein said second and thirdtransmissivities differ in relative phase.
 22. The semiconductor patternmask of claim 21 wherein each of said second features is at leastsimilar in size to said first features, said second features formingstructures in said semiconductor pattern.
 23. The semiconductor patternmask of claim 21 wherein each of said second features is smaller thanany of said first features, said second features not forming structuresin said semiconductor pattern.
 24. The semiconductor pattern mask ofclaim 1 wherein at least a subset of said first features are in a densenonuniform pattern.
 25. The semiconductor pattern mask of claim 24wherein substantially all of said first features are in said densenonuniform pattern.
 26. The semiconductor pattern mask of claim 24wherein: said third transmissivity is substantially equal to said secondtransmissivity; each of said first features has dimensions selected tobe imaged by light having a predetermined wavelength; and each of saidsecond features has dimensions substantially less than saidpredetermined wavelength, said second features not forming structures insaid semiconductor pattern.
 27. The semiconductor pattern mask of claim26 wherein each of said second features has dimensions of between aboutone-third and about one-half of said predetermined wavelength.
 28. Thesemiconductor pattern mask of claim 24 wherein said second and thirdtransmissivities differ in relative phase.
 29. The semiconductor patternmask of claim 28 wherein each of said second features is at leastsimilar in size to said first features, said second features formingstructures in said semiconductor pattern.
 30. The semiconductor patternmask of claim 28 wherein each of said second features is smaller thanany of said first features, said second features not forming structuresin said semiconductor pattern.
 31. A semiconductor manufacturing methodcomprising: providing a semiconductor substrate having a photosensitivesurface; providing a semiconductor pattern mask comprising: a masksubstrate characterized by a first transmissivity to light, a pluralityof first features on said mask substrate for forming structures in asemiconductor pattern, each of said first features being characterizedby a second transmissivity to light, and a plurality of second featureson said mask substrate, each of said second features being characterizedby a third transmissivity to light; exposing said photosensitive surfaceto light through said mask; and processing said exposed photosensitivesurface to develop said structures on said semiconductor substrate. 32.The method of claim 31 wherein said first transmissivity issubstantially opaque and said second transmissivity is substantiallytransmissive.
 33. The method of claim 31 wherein said firsttransmissivity is substantially transmissive and said secondtransmissivity is substantially opaque.
 34. The method of claim 31wherein: said third transmissivity is substantially equal to said secondtransmissivity; and each of said second features is smaller than any ofsaid first features, said second features not forming structures in saidsemiconductor pattern.
 35. The method of claim 34 wherein: each of saidfirst features has dimensions selected to be imaged by light having apredetermined wavelength; and each of said second features hasdimensions substantially less than said predetermined wavelength. 36.The method of claim 35 wherein each of said second features hasdimensions of between about one-third and about one-half of saidpredetermined wavelength.
 37. The method of claim 31 wherein said secondand third transmissivities differ in relative phase.
 38. The method ofclaim 37 wherein each of said second features is at least similar insize to said first features, said second features forming structures insaid semiconductor pattern.
 39. The method of claim 37 wherein each ofsaid second features is smaller than any of said first features, saidsecond features not forming structures in said semiconductor pattern.40. The method of claim 31 wherein at least a subset of said firstfeatures are in a nonuniform repetitive pattern.
 41. The method of claim40 wherein said nonuniform repetitive pattern is dense.
 42. The methodof claim 40 wherein: said third transmissivity is substantially equal tosaid second transmissivity; said exposing comprises exposing saidphotosensitive surface to light having a predetermined wavelength; andeach of said second features has dimensions substantially less than saidpredetermined wavelength, said second features not forming structures insaid semiconductor pattern.
 43. The method of claim 42 wherein each ofsaid second features has dimensions of between about one-third and aboutone-half of said predetermined wavelength.
 44. The method of claim 40wherein said second and third transmissivities differ in relative phase.45. The method of claim 44 wherein each of said second features is atleast similar in size to said first features, said second featuresforming structures in said semiconductor pattern.
 46. The method ofclaim 44 wherein each of said second features is smaller than any ofsaid first features, said second features not forming structures in saidsemiconductor pattern.
 47. The method of claim 40 wherein substantiallyall of said first features are in said nonuniform repetitive pattern.48. The method of claim 47 wherein said nonuniform repetitive pattern isdense.
 49. The method of claim 47 wherein: said third transmissivity issubstantially equal to said second transmissivity; said exposingcomprises exposing said photosensitive surface to light having apredetermined wavelength; and each of said second features hasdimensions substantially less than said predetermined wavelength, saidsecond features not forming structures in said semiconductor pattern.50. The method of claim 49 wherein each of said second features hasdimensions of between about one-third and about one-half of saidpredetermined wavelength.
 51. The method of claim 47 wherein said secondand third transmissivities differ in relative phase.
 52. The method ofclaim 51 wherein each of said second features is at least similar insize to said first features, said second features forming structures insaid semiconductor pattern.
 53. The method of claim 51 wherein each ofsaid second features is smaller than any of said first features, saidsecond features not forming structures in said semiconductor pattern.54. The method of claim 31 wherein at least a subset of said firstfeatures are in a dense nonuniform pattern.
 55. The method of claim 54wherein substantially all of said first features are in said densenonuniform pattern.
 56. The method of claim 54 wherein: said thirdtransmissivity is substantially equal to said second transmissivity;said exposing comprises exposing said photosensitive surface to lighthaving a predetermined wavelength; and each of said second features hasdimensions substantially less than said predetermined wavelength, saidsecond features not forming structures in said semiconductor pattern.57. The method of claim 56 wherein each of said second features hasdimensions of between about one-third and about one-half of saidpredetermined wavelength.
 58. The method of claim 54 wherein said secondand third transmissivities differ in relative phase.
 59. The method ofclaim 58 wherein each of said second features is at least similar insize to said first features, said second features forming structures insaid semiconductor pattern.
 60. The method of claim 58 wherein each ofsaid second features is smaller than any of said first features, saidsecond features not forming structures in said semiconductor pattern.